pythoncapi
diff --git a/‎Lib/test/support/__init__.py‎
Lines changed: 55 additions & 12 deletions b/‎Lib/test/support/__init__.py‎
Lines changed: 55 additions & 12 deletions
diff --git a/‎Lib/test/test_idle.py‎
Lines changed: 1 addition & 13 deletions b/‎Lib/test/test_idle.py‎
Lines changed: 1 addition & 13 deletions
diff --git a/‎Lib/test/test_tk.py‎
Lines changed: 1 addition & 2 deletions b/‎Lib/test/test_tk.py‎
Lines changed: 1 addition & 2 deletions
diff --git a/‎Lib/test/test_ttk_guionly.py‎
Lines changed: 1 addition & 2 deletions b/‎Lib/test/test_ttk_guionly.py‎
Lines changed: 1 addition & 2 deletions
diff --git a/‎Lib/tkinter/test/support.py‎
Lines changed: 2 additions & 44 deletions b/‎Lib/tkinter/test/support.py‎
Lines changed: 2 additions & 44 deletions
diff --git a/‎Misc/NEWS‎
Lines changed: 4 additions & 0 deletions b/‎Misc/NEWS‎
Lines changed: 4 additions & 0 deletions
@@ -378,12 +378,16 @@ def forget(modname):
         unlink(importlib.util.cache_from_source(source, debug_override=True))
         unlink(importlib.util.cache_from_source(source, debug_override=False))
 
-# On some platforms, should not run gui test even if it is allowed
-# in `use_resources'.
-if sys.platform.startswith('win'):
-    import ctypes
-    import ctypes.wintypes
-    def _is_gui_available():
+# Check whether a gui is actually available
+def _is_gui_available():
+    if hasattr(_is_gui_available, 'result'):
+        return _is_gui_available.result
+    reason = None
+    if sys.platform.startswith('win'):
+        # if Python is running as a service (such as the buildbot service),
+        # gui interaction may be disallowed
+        import ctypes
+        import ctypes.wintypes
         UOI_FLAGS = 1
         WSF_VISIBLE = 0x0001
         class USEROBJECTFLAGS(ctypes.Structure):
@@ -403,10 +407,49 @@ class USEROBJECTFLAGS(ctypes.Structure):
             ctypes.byref(needed))
         if not res:
             raise ctypes.WinError()
-        return bool(uof.dwFlags & WSF_VISIBLE)
-else:
-    def _is_gui_available():
-        return True
+        if not bool(uof.dwFlags & WSF_VISIBLE):
+            reason = "gui not available (WSF_VISIBLE flag not set)"
+    elif sys.platform == 'darwin':
+        # The Aqua Tk implementations on OS X can abort the process if
+        # being called in an environment where a window server connection
+        # cannot be made, for instance when invoked by a buildbot or ssh
+        # process not running under the same user id as the current console
+        # user.  To avoid that, raise an exception if the window manager
+        # connection is not available.
+        from ctypes import cdll, c_int, pointer, Structure
+        from ctypes.util import find_library
+
+        app_services = cdll.LoadLibrary(find_library("ApplicationServices"))
+
+        if app_services.CGMainDisplayID() == 0:
+            reason = "gui tests cannot run without OS X window manager"
+        else:
+            class ProcessSerialNumber(Structure):
+                _fields_ = [("highLongOfPSN", c_int),
+                            ("lowLongOfPSN", c_int)]
+            psn = ProcessSerialNumber()
+            psn_p = pointer(psn)
+            if (  (app_services.GetCurrentProcess(psn_p) < 0) or
+                  (app_services.SetFrontProcess(psn_p) < 0) ):
+                reason = "cannot run without OS X gui process"
+
+    # check on every platform whether tkinter can actually do anything
+    if not reason:
+        try:
+            from tkinter import Tk
+            root = Tk()
+            root.destroy()
+        except Exception as e:
+            err_string = str(e)
+            if len(err_string) > 50:
+                err_string = err_string[:50] + ' [...]'
+            reason = 'Tk unavailable due to {}: {}'.format(type(e).__name__,
+                                                           err_string)
+
+    _is_gui_available.reason = reason
+    _is_gui_available.result = not reason
+
+    return _is_gui_available.result
 
 def is_resource_enabled(resource):
     """Test whether a resource is enabled.  Known resources are set by
@@ -421,7 +464,7 @@ def requires(resource, msg=None):
     executing.
     """
     if resource == 'gui' and not _is_gui_available():
-        raise unittest.SkipTest("Cannot use the 'gui' resource")
+        raise ResourceDenied(_is_gui_available.reason)
     # see if the caller's module is __main__ - if so, treat as if
     # the resource was set
     if sys._getframe(1).f_globals.get("__name__") == "__main__":
@@ -1589,7 +1632,7 @@ def _id(obj):
 
 def requires_resource(resource):
     if resource == 'gui' and not _is_gui_available():
-        return unittest.skip("resource 'gui' is not available")
+        return unittest.skip(_is_gui_available.reason)
     if is_resource_enabled(resource):
         return _id
     else:
 
@@ -1,24 +1,12 @@
 import unittest
 from test import support
-from test.support import import_module, use_resources
+from test.support import import_module
 
 # Skip test if _thread or _tkinter wasn't built or idlelib was deleted.
 import_module('threading')  # imported by PyShell, imports _thread
 tk = import_module('tkinter')  # imports _tkinter
 idletest = import_module('idlelib.idle_test')
 
-# If buildbot improperly sets gui resource (#18365, #18441), remove it
-# so requires('gui') tests are skipped while non-gui tests still run.
-# If there is a problem with Macs, see #18441, msg 193805
-if use_resources and 'gui' in use_resources:
-    try:
-        root = tk.Tk()
-        root.destroy()
-        del root
-    except tk.TclError:
-        while 'gui' in use_resources:
-            use_resources.remove('gui')
-
 # Without test_main present, regrtest.runtest_inner (line1219) calls
 # unittest.TestLoader().loadTestsFromModule(this_module) which calls
 # load_tests() if it finds it. (Unittest.main does the same.)
 
@@ -6,8 +6,7 @@
 support.import_fresh_module('tkinter')
 
 # Skip test if tk cannot be initialized.
-from tkinter.test.support import check_tk_availability
-check_tk_availability()
+support.requires('gui')
 
 from tkinter.test import runtktests
 
 
@@ -9,8 +9,7 @@
 support.import_fresh_module('tkinter')
 
 # Skip test if tk cannot be initialized.
-from tkinter.test.support import check_tk_availability
-check_tk_availability()
+support.requires('gui')
 
 from _tkinter import TclError
 from tkinter import ttk
 
@@ -1,52 +1,10 @@
 import sys
 import tkinter
 import unittest
-
-_tk_unavailable = None
-
-def check_tk_availability():
-    """Check that Tk is installed and available."""
-    global _tk_unavailable
-
-    if _tk_unavailable is None:
-        _tk_unavailable = False
-        if sys.platform == 'darwin':
-            # The Aqua Tk implementations on OS X can abort the process if
-            # being called in an environment where a window server connection
-            # cannot be made, for instance when invoked by a buildbot or ssh
-            # process not running under the same user id as the current console
-            # user.  To avoid that, raise an exception if the window manager
-            # connection is not available.
-            from ctypes import cdll, c_int, pointer, Structure
-            from ctypes.util import find_library
-
-            app_services = cdll.LoadLibrary(find_library("ApplicationServices"))
-
-            if app_services.CGMainDisplayID() == 0:
-                _tk_unavailable = "cannot run without OS X window manager"
-            else:
-                class ProcessSerialNumber(Structure):
-                    _fields_ = [("highLongOfPSN", c_int),
-                                ("lowLongOfPSN", c_int)]
-                psn = ProcessSerialNumber()
-                psn_p = pointer(psn)
-                if (  (app_services.GetCurrentProcess(psn_p) < 0) or
-                      (app_services.SetFrontProcess(psn_p) < 0) ):
-                    _tk_unavailable = "cannot run without OS X gui process"
-        else:   # not OS X
-            import tkinter
-            try:
-                tkinter.Button()
-            except tkinter.TclError as msg:
-                # assuming tk is not available
-                _tk_unavailable = "tk not available: %s" % msg
-
-    if _tk_unavailable:
-        raise unittest.SkipTest(_tk_unavailable)
-    return
+from test.support import requires
 
 def get_tk_root():
-    check_tk_availability()     # raise exception if tk unavailable
+    requires('gui')             # raise exception if tk unavailable
     try:
         root = tkinter._default_root
     except AttributeError:
 
@@ -259,6 +259,10 @@ Documentation
 Tests
 -----
 
+- Issue #18604: Consolidated checks for GUI availability.  All platforms now
+  at least check whether Tk can be instantiated when the GUI resource is
+  requested.
+
 - Issue #21275: Fix a socket test on KFreeBSD.
 
 - Issue #21223: Pass test_site/test_startup_imports when some of the extensions